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TYPE CA-5 RELAY
When the switch closes, the moving contacts bridge
two stationary contacts completing the trip circuit.
Also during the operation two fingers on the armature
deflect a spring located on the front of the switch
which allows the operation indicator target to drop.
The target is reset from the outside of the case by a
push rod located at the bottom of the cover.
A core screw accessible from the top of the switch
provides the adjustable pickup range. The minimum
and maximum pickup points are indicated on the scale,
which is located to the rear of the core screw.
OPERATION
Fig. 3 shows the current through the relay for
external and internal fault conditions. With the relay
connected as in Fig. 3A, a through fault causes cur
rents to flow through the two restraining windings in
the same direction. If the main current transformers
operate properly, these restraining currents are equal,
and no current flows in the auxiliary current transformer
and operating coil winding. In this case, there will be
only contact opening torque produced.
In the case of a heavy internal fault, when an
external source feeds current into the fault, the re
straining currents are in opposite directions, and re
straining torque tends to cancel out
When the currents fed from the two sides are equal,
the restraint is totally cancelled. When unequal cur
rents flow in from the two sides, the restraint is
equivalent to the difference in the two currents. In
this case, the currents in the restraining windings will
add together and go through the operating winding,
producing
a contact closing torque. The operating
curves for the relay are shown in Figs. 4 and 5.
This relay has variable percentage characteristics
which means that the operating coil current required
to close the relay contact, expressed in percent of
the total restraint current, varies with the magnitude
of the restraint current. The relay sensitivity is high,
corresponding to a low percentage ratio, at light cur
rents, and its sensitivity is low, corresponding to
high percentage unbalance, at high currents. The relay
is made sensitive at low curr e nts in order that it will
detect light internal faults. At the same time, however,
its reduced sensitivity at the higher currents allows
the various current transformers involved to depart
from their true ratio to a large extent without causing
false tripping of the relay for external faults.
CH ARACTERI STI CS
The type CA-5 percentage differential relay is
available in two designs: a relay with a minimum trip
current of 0.14 amp. for generator protection and a
relay with a range of adjustment of minimum trip cur
rent from 0.50 amp. to 2.0 amp. for transformer protec
tion.
The time of operation of the generator relay is
shown in Fig. 7, and the time of operation of the
transformer relay is shown in Fig. 6. The percentage
slope characteristics are shown in Fig. 5 (Generator
relay) and Fig. 4 (Transformer relay).
Trip Circuit
The main contacts will safely close 30 amperes
at 250 volts d-e and the seal-in contacts of the indicat
ing contactor switch will safely carry this current
long enough to trip a circuit breaker.
The indicating instantaneous trip contacts will
safely close 30 amperes at 250 volts d-e, and will
carry this current long enough to trip a breaker.
The indicating contactor switch has two taps
that provide a pickup setting of 0.2 or 2 amperes.
To change taps requires connecting the lead
located in front of the tap block to the desired setting
by means of a screw connection.
as
in Fig. 3B.
Trip Circuit Constant
Indicating Contactor Switch (ICS)
Restraint Coil Circuit
Continuous Rating
1 Second Rating
Volt Amperes
Power Factor
Operating Coil Circuit
Continuous Rating
1 Second Rating
Volt Amperes
No calculations are required to set the CA-5 relay.
0.2 ampere tap 6.5 ohms d-e resistance
2.0 ampere tap 0 .15 ohms d-e resistance
ENERG Y REQUIREMENTS
@
5 amperes
SETTING CALCULATIONS
10 amperes
250 amperes
. 75
0.7 lag
5 amperes
150 amperes
Variable, see Fig.8
3
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